1. Field of the Invention
The present invention relates to a chain guide structure for a chain drive type transmission mechanism by which an output of an internal combustion engine mounted on a vehicle is transmitted to a rear wheel through a drive chain.
2. Description of Background Art
In a chain drive type transmission mechanism, in order that a drive chain wound around a drive sprocket fitted to an output shaft of an internal combustion engine will not be disengaged from the drive sprocket due to, for example, vibrations during operation of the vehicle, a chain guide is provided so as to cover the drive chain on the drive sprocket over a range from a front side of a radial periphery of the drive sprocket to an obliquely upper rear side of the periphery.
The chain guide is provided in proximity to the drive chain wound around the drive sprocket. At its rear-side portion, the chain guide restrains vibrations of the drive chain in a position where the drive chain starts being wound around the drive sprocket. At its front-side portion, the chain guide is situated closest to the drive chain, thereby preventing the drive chain from being disengaged from the drive sprocket.
In general, a chain guide is mounted to an engine case by being fastened to fastening boss portions of the engine case.
In the case of a vehicle designed for traveling on rough terrain, such as an off-road vehicle, a small stone or the like may spring up and may get nipped between a drive chain and a chain guide in the manner of being dragged in by the drive chain which is rotationally moving. In view of this, a large cover may be provided for preventing a small stone or the like from getting nipped between the drive chain and the chain guide. Or a contrivance may be made such that the impact load on the chain guide can be received in a satisfactory manner. See, for example, Japanese Patent Laid-Open No. Hei 6-255559.
In the chain guide structure disclosed in Japanese Patent Laid-Open No. Hei 6-255559, the chain guide is put in contact with a required part of a crankcase of an internal combustion engine.
Even if a small stone or the like gets nipped between the chain guide and a drive chain wound around a drive sprocket and an impact is exerted on the chain guide, the impact load can be received by the required part of the crankcase, and the small stone or the like can be eliminated.
However, depending on the conditions of the rough terrain, there may be cases where a small stone exceeding an anticipated size gets nipped between the chain guide surface and the drive chain. In such a case, if the impact load received by the chain guide is considerably high, the strong impact force received by the chain guide may possibly be exerted as a high load on a required part of a crankcase in contact with the chain guide, thereby damaging the required part together with fastening boss portions. Such a possibility should, naturally, be excluded.